1. Field of the Invention
This invention relates generally to a communications system and, more particularly, to a load shedding method to enhance uplink margin with combined FDMA/TDMA uplinks in a satellite based cellular communications system.
2. Discussion of the Related Art
In satellite based cellular communications systems, a central terrestrial control processor or network operations center (NOC) generally controls one or more communications satellites operating within the communications system. Each communications satellite within the communications system services multiple users located in multiple geographic areas, known as ground cells. The communications satellites receive and transmit data signals to and from the multiple users or terrestrial terminals positioned at the different locations within the separate ground cells on a point-to-point manner. In a frequency division multiple access (FDMA)/time division multiple access (TDMA) communications system, signals on the same frequency, polarization and time slot are generally reused by different users within the spatially separated ground cells because of bandwidth constraints.
An antenna on each communications satellite generates a multitude of spot beams to illuminate a surface on the earth where the ground cells are located in order to accommodate the re-use of the frequencies throughout the communications system. Antenna patterns for each spot beam covering each ground cell typically roll off very fast towards the edge of the beam, and thus with even small antenna or satellite pointing errors, the user on the ground may see significant signal attenuation due to this mispointing. Moreover, since modern satellite communications systems now operate at much higher carrier frequencies (for example, Ka-band), these signals are vulnerable to large attenuations due to rain, scintillation, and other atmospheric effects. Because of this, the terrestrial user terminals will have degraded performance unless attenuation mitigation is employed.
Conventional ways of mitigating attenuation caused by atmospheric effects, roll-off, etc., include transmitting at a lower data rate or at a higher power level to enhance or increase the transmitted energy per bit. Alternatively, additional error control coding may be used where the coding rate is increased without increasing the occupied bandwidth. This, however, reduces the amount of information bits being transmitted since more error control bits are generated with the higher error control rate. Increasing the coding also causes the demodulator on-board the satellite to be more complex. Increased power also means that the transmitter amplifier is oversized in nonfade conditions, thereby increasing overall system cost without any tangible benefits.
What is needed then is a load shedding method and apparatus to enhance uplink margin which does not suffer from the above-mentioned disadvantages. This will, in turn, provide a satellite based cellular communications system which utilizes multiple FDMA carrier channels simultaneously to operate in a multi-carrier fashion; enable "shedding" of carrier channels as fading or attenuation levels increase, thereby increasing the transmitted energy per bit to overcome the increased attenuation; provide a means for monitoring and controlling the appropriate number of carrier channels to be used by the terrestrial terminal; eliminate the requirement of multiple rate demodulators on-board the satellite to reduce satellite complexity; and provide terrestrial terminals that can support different numbers of carrier channels to provide varying fade capability such that the terrestrial terminals are scaleable in size and cost. It is, therefore, an object of the present invention to provide a satellite based cellular communications system which utilizes a load shedding method to enhance uplink margin with combined FDMA/TDMA uplinks.